In recent years, attentions have been paid to the PLC (Power Line Communication) system. The PLC system is a data transmission system which bidirectionally connects each home and a relay station to the Internet or the like provided in an electric substation or an electric pole using a power line of the commercial power source and bidirectionally connects a communication network and each electric appliance using a power line of the commercial power source in a home. As the PLC system, the one which performs communications according to OFDM (Orthogonal Frequency Division Multiplexing) transmission method using a frequency in the band of, for example, about 30 MHz or lower has been proposed.
In Japanese Patent Application Laid-Open Publication No. 2005-051768 described below, a PLC system using the OFDM transmission method has been disclosed. In this system, in order to prevent the electromagnetic radiation generated by a PLC signal transmitted through a power supply line from interfering with a broadcasting RF signal radiated in the same frequency band, the PLC node scans an entire frequency range designated for the power line communication so as to detect a frequency band occupied by the broadcasting service. Then, the frequency band for the power line communication excluding the above-described frequency band is allocated.
Also, Japanese Patent Application Laid-Open Publication No. 2003-188781 described below has disclosed a power line carrier noise resistant communication system in which the noise resistant characteristics are improved so that stable data communication can be performed. This system comprises: means for diluting the influence of the discontinuous noise by distributing signals to a time axis and a frequency axis by Walsh transform and OFDM modulation and demodulation as a transmission circuit and a receiving circuit in a power line carrier communication apparatus; and means for detecting the noise not diluted by the above means by wavelet analysis, thereby cutting the noise.
In this system, the former means deals with a relatively small discontinuous noise and both the former and latter means deal with a relatively large discontinuous noise, whereby the discontinuous noises of arbitrary sizes are reduced. Also, by disposing the power line carrier communication apparatus at an arbitrary place of household wiring such as power line laid in advance in a home or an office and transmitting signals via a coupling circuit in the power line carrier communication apparatus, data communications are carried out between the power line carrier communication apparatuses or with an external communication apparatus.
FIG. 17 is a block diagram showing an example of the configuration of the PLC system in a home. A power line (AC 100 V) is connected to an indoor wiring 304 through a low pass filter 303 which allows electricity of not more than several hundreds Hz to pass through. Also, a master MODEM 302 is connected to the indoor wiring 304 and the master MODEM 302 having a router function is connected to the Internet (server) through an optical terminating device 301, an optical fiber line 330 and others.
The indoor wiring 304 of an ordinary home is branched to an arbitrary number of lines at arbitrary positions (length) and connected to respective load apparatuses A (310) to D (322) through an outlet 305 or a switch 306. The load apparatus includes, for example, various apparatuses using electric power such as a personal computer, a TV set, a refrigerator, a luminaire, an air conditioner, a vacuum cleaner and the like. Some of these load apparatuses such as the personal computer are equipped with a slave MODEM 311 and performs data communication with the master MODEM 302 or other slave MODEM using a frequency in the band of, for example, about 30 MHz or lower through the indoor wiring 304.
The indoor wiring 304 can be regarded as a pair cable type transmission line in the data transmission, and the indoor wiring 304 is branched to plural lines and each line length varies by connecting/disconnecting a load apparatus to/from the outlet. Further, the line length and the terminating impedance are changed also by turning on/off a switch of any load apparatus.
Therefore, reflection and resonance of signals occur due to the line length up to a branch point or a terminal point and the terminating impedance, so that a spectrum null point in which signals of a specified frequency are not transmitted at all is generated and the frequency of the null point is frequently changed depending on the condition of the load apparatus. Further, when the null point is generated, a problem that an equalizer diverges and cannot perform equalization is caused.
The above-described conventional PLC system uses the OFDM transmission method, but the OFDM transmission method is a block transmission method and the roll-off of a transmission pulse needs to be decreased for increasing the use efficiency of a frequency band. However, if the roll-off is decreased, a guard interval signal to be applied before/after a block is extended, and the problem that the transmission efficiency is lowered is caused.
Also, although a deep notch characteristic which largely suppresses only a specified band is desired to be realized in the PLC system, since the OFDM transmission method is a block transmission method, this deep notch characteristic cannot be realized in the vicinity of a border of adjoining blocks. Further, the notch filter needs to be attached to a latter stage of the OFDM if it is intended to realize the notch for suppressing the electromagnetic radiation accurately, so that the resulting long transient response affects the adjoining blocks and it is difficult to erase this influence. Further, if it is intended to realize the notch for removing an external noise, the data distribution to corresponding sub channels of the OFDM has to be stopped, so that such a complicated process as rebuilding of the block is necessary.
In the embodiment of the present invention described later, a precoder is used. This precoder has been studied as one type of the equalizer corresponding to an uneven characteristic of a communication transmission channel.
According to M. TOMLINSON, “NEW AUTOMATIC EQUALISER EMPLOYING MODULO ARITHMETIC”, ELECTRONICS LETTERS, Mar. 25, 1971, Vol. 7, No. 5/6, pp. 138-139, this equalizer performs the pre-equalization by providing an IIR (Infinite Impulse Response) filter having an inverse characteristic of impulse response of the transmission channel on a sending side, and at this time, by replacing the addition with the modulo N operation, it becomes possible to prevent the output of the IIR filter from being increased and unstable.
Further, according to HIROSHI HARASHIMA, HIROSHI MIYAKAWA, “Matched-Transmission Technique for Channels With Intersymbol Interference” IEEE TRANSACTIONS ON COMMUNICATIONS, vol. COM-20, No. 4, August 1972, pp. 774-780, the similar method can be obtained by generalizing the technology for removing intersymbol interference of a narrow band transmission channel, which is conventionally referred to as partial response or correlation level symbol.
However, although the above-described impulse response is based on the premise that the initial response under the delay time 0 is set to 1, the impulse response of the communication transmission channel takes a vibration waveform which starts with a forerunner having a small amplitude, gradually increases and then attenuates. Consequently, in the above-described pre-equalization of the IIR filter (corresponding to “precoder” in this specification), the number of levels of the reception signal extremely increases, and thus the practical use thereof cannot be achieved without any special treatment. Further, the pre-equalization technology in the above-mentioned documents is just a technology for equalizing the transmission channel.